Conventional interior rearview mirrors for vehicles typically include a prismatic substrate having a first surface facing a viewer of the mirror who is seated in the vehicle and a second, reflecting surface (typically silver coated) opposite the first surface. Electrically variable reflectance mirrors may comprise electro-optic reflective element assemblies, which comprise a first or front substrate and a second or rear substrate (typically flat glass substrates) with an electro-optic medium (typically an electrochromic medium) disposed between the front substrate and the rear substrate. Visible light transmission through the electro-optic medium is controllable by applying an electrical voltage thereto. Such a twin substrate electro-optic element has four surfaces; a first surface frontmost on the front or first substrate and a second surface opposing the first surface and also on the first substrate. The rear or second substrate has a third surface and an opposing fourth surface. The second surface of the front substrate is commonly coated with a transparent electrical conductor, such as indium tin oxide. The third surface of the rear substrate is commonly coated with a metallic reflecting/electrical conductor, such as a layer of silver or of a silver alloy or a stack layer of ITO/Ag/ITO or the like, or it may be coated with a transparent electrical conductor, such as indium tin oxide, while the opposing surface of the second or rear substrate (the fourth surface) may be coated with a mirror reflector, such as silver metal layer. To form an electro-optical mirror element, the second surface of the front substrate is arranged opposing the third surface of the rear substrate and the electro-optic medium is disposed between the second surface of the front substrate and the third surface of the rear substrate. It is common to include a display device at the mirror assembly, such as rearward of the second or rear substrate and so behind its fourth surface so that its display image is viewable through the substrates by a person seated within the interior cabin of the vehicle (such as a driver of the vehicle) viewing the interior rearview mirror assembly when it is normally mounted to the windshield or to the header of the vehicle.
It is thus known to have a display device, such as a video display device capable of displaying a video image captured by a video imaging device, such as a video camera, disposed behind and visible through an interior rearview mirror element, such as, for example, displays of the types disclosed in U.S. Pat. Nos. 5,668,663; 5,724,187; 5,956,181 and 6,690,268, which are hereby incorporated herein by reference in their entireties. It is thus known to have a covert display device (a.k.a. a display-on-demand display device), such as a covert video display device, disposed behind and thus visible through a transflective interior rearview mirror element, such as an electrochromic transflective interior rearview mirror element, such as the types described in U.S. Pat. Nos. 5,668,663; 5,724,187 and 6,690,268, which are hereby incorporated herein by reference in their entireties.
It has also been suggested to use a polymeric transflective film or layer or element at the fourth surface of a rearview mirror reflective element. For example, it has been suggested that an all-polymeric film or element that is both significantly reflecting and substantially transmitting, such as PRM material such as described in U.S. Pat. No. 5,724,187, which is hereby incorporated herein by reference in its entirety, or a polymer-based film having reflective properties, such as the multilayer reflective film described in U.S. Pat. No. 6,352,761; and/or U.S. Pat. Pub. No. US 2005/0174645, published Aug. 11, 2005 which are hereby incorporated herein by reference in their entireties, be used as a mirror reflector in an automotive rearview mirror. Such an all-polymeric specular mirror film is available from 3M of Minneapolis, Minn. under the tradename Vikuiti™ Enhanced Specular Reflector Film (ESR), which consists of over 300 polymers layers of different refractive (typically alternating) indexes that create a mirror finish.
However, successful commercialization of covert video displays that are disposed behind and whose presence behind the mirror reflector is principally visible/discernible to the driver or other occupant of the vehicle only when the display image is actuated has hitherto been impaired by display image washout during higher ambient lighting conditions, such as daylight lighting conditions, particularly on a sunny day. Therefore, there is a need in the art for an improved covert video imaging display device for an interior rearview mirror assembly of a vehicle that overcomes shortcomings of the prior art.